1. Field of the Invention
This invention relates to a route guidance apparatus such as a navigation system.
2. Pelated Art
Navigation systems which detect a present position of a vehicle by means of GPS or the like as the vehicle travels and display that position on a display together with a road map, thereby allowing the vehicle to reach a destination smoothly, are known. Also, navigation systems which obtain a suitable route from the present position to the destination and provide a route guidance are known, and contribute to smoother drives.
Among navigation systems as described above, those which, every time a vehicle reaches a turning point on a route for which guidance is to be provided to the user, provide travel guidance for that turning point with a voice are known. For example those which provide voice guidance as to the distance to a turning point and a direction in which to turn there, like ".largecircle..largecircle. meters ahead, right turn.", and even provide guidance including the name of the intersection, like ".DELTA..DELTA. meters ahead, .tbd..tbd. Intersection, left turn." are known. By informing the driver of the travel guidance with good timing with a voice in this way, it is not necessary for the user to frequently pay attention to the positional relationship between the present position and the turning point and so on, and thus it is extremely convenient.
As one road system often seen mainly in Europe and America, there is the traffic circle (Traffic Circle: circular intersection, also called a rotary or roundabout). This traffic circle, an example of which is shown in FIG. 7A, is often made up of a plurality of entry roads 102 and a plurality of exit roads 103 connected to a closed circular route 101. For example, when a route guidance apparatus provides voice guidance about a route entering the traffic circle from the direction shown with the arrow A in FIG. 7A and exiting in the direction shown with the arrow B, after the vehicle enters the traffic circle, it is indicated how many ahead is the exit road 103 to be taken. Therefore, in the case of FIG. 7A, because the exit road 103 shown with 2 is to be taken, voice guidance is provided like "Please exit on the exit road two ahead.". On the other hand, in the case of a traffic circle having inner side exit roads, as shown in FIG. 7B, when the route guidance apparatus provides voice guidance about a route entering from the direction shown with the arrow A and exiting in the direction shown with the arrow B, because the exit road 103 shown with 3 is to be taken, voice guidance is provided like "Please exit on the exit road three ahead.".
However, among traffic circles actually existing, those having exit roads set on the circle outer side only as shown in FIG. 7A are the vast majority, and traffic circles of a shape having exit roads set on the inner side of the circle also, as shown in FIG. 7B, do not even account for ten percent. Accordingly, as the consciousness of the driver, often it is taken for granted that exit roads are roads exiting to the outer side, and the possibility of missing exit roads to the inner side is high. Therefore, after the vehicle enters the traffic circle, even through it is indicated how many ahead the exit road 103 that is to be taken is, there is a possibility of the vehicle exiting from the wrong exit road 103. That is, in the case shown in FIG. 7B, when the vehicle exits in the direction shown with the arrow B as mentioned above, because the vehicle passes through the exit road 103 shown with 3, the voice guidance "Please exit on the exit road three ahead." is provided. But if attention is only paid to the outer side exit roads 103 then "the exit road three ahead" becomes the exit road 103 shown with 4. As a result, the problem arises that the driver goes past the exit road 103 of 3 on which he should exit and exits on the exit road 103 of 4.
In the meantime, as a route guidance in a conventional map displaying apparatus, normally, a guide route from a present position to a destination is additionally displayed on a map displaying the present position's vicinity of the vehicle. In that display method, by the color of the guide route being made different from the normal road color, it is made easy to see it. Also, at a turning point (here, for example, an intersection) at which a turn should be made on that guide route an enlarged intersection map is displayed to make it possible for the driver to grasp the intersection at which he is to turn next more correctly. In this enlarged intersection map, the intersection and its vicinity map are enlarged and displayed on the screen. The enlarged map has been drawn on a fixed scale such as for example 1/500 and displayed on the display.
As described above, the traffic circle, for example as shown in FIGS. 14A, 14C is made up of a plurality of entrance roads and a plurality of exit roads connected to a closed circular route, and its size (for example the maximum diameter of the circular route) is various. For example the traffic circle shown in FIG. 14A is one of which the circular route part is relatively small. When this is enlarged on a fixed scale as conventional and displayed, there are times when as shown in FIG. 14B a line collapses and it becomes impossible for the user to distinguish the details of the traffic circle depending on the link widths drawn and the like.
On the other hand, the traffic circle shown in FIG. 14C is one of which the circular route part is relatively large. When this is enlarged on the fixed scale and displayed, in this case, it does not happen that the line collapses as shown in FIG. 14B and it becomes possible for the user to distinguish the details of the traffic circle. However, there are cases wherein as shown in FIG. 14D only a part of the traffic circle is displayed. This is because since the connection part between the circular route of the traffic circle and the exit road is taken as an intersection, only the vicinities thereof are displayed. However, when the vehicle travels on the traffic circle, after entering the circular route, it is necessary for the driver to travel recognizing how many ahead the exit road at which a turn should be made is. That is, unless not only the exit road alone but also the relative positional relationships in the entire traffic circle are understood to the driver, there is a possibility of not being able to distinguish which exit road to exit on.
Also, the same kind of problem arises also for loop-shaped connecting roads which connect ground roads and elevated roads in three-dimensional intersection structures. For example, the loop-shaped connecting road is a road such as a so-called rampway through which the vehicle passes when the vehicle proceeds from an ordinary road which is a "ground road" to an expressway which is an "elevated road" (or a Freeway seen in Europe and America).
For example, the loop-shaped connecting road shown in FIG. 17A is one whose loop-shaped part is relatively large. When this is enlarged on the fixed scale and displayed, as shown in FIG. 11B there are cases wherein only a part of the loop shape is displayed. This is because, since vehicles cannot actually pass through the three-dimensional intersection part between the ground road and the elevated road, the loop-shaped connecting road is not taken as an intersection between ordinary roads and it is only displayed as a diagonal branch at the entry part to the loop-shaped connecting road. However, to a driver wanting to proceed to a road connected to the elevated road, there is a possibility of it not being possible to judge whether or not the diagonal branch displayed as shown in FIG. 17B is really a loop-shaped connecting road. In the end, the driver may not understand which branch to turn at.
The "loop-shaped connecting road" referred to here indicates a road, among connecting roads connecting ordinary roads which are "ground roads" and expressways and Freeways which are "elevated roads", having a loop shape such that difference in road direction from the entry angle to the connecting road is equal to or greater than a predetermined angle. As the predetermined angle constituting the condition of the "loop-shaped connecting road", for example about 180 degrees is conceivable. This is because, when the road direction changes more than about 180 degrees, the possibility of it being impossible to judge whether or not a diagonal branch displayed as in FIG. 17B is really a loop-shaped connecting road increases. However, even when the difference of the road direction is less than 180 degrees, if the connecting road has a shape such that the above-mentioned problem arises, that connecting road can also be regarded as a loop-shaped connecting road.
Also, in relation to this kind of problem not only the traffic circles and loop-shaped connecting roads discussed above but also complex intersections must be similarly considered. For example, when a road of a small width and a road of a large width make a "diagonal T intersection" or a "diagonal crossing intersection" and also roads of large widths including those kinds of intersections make a "diagonal crossroads intersection", it is a situation such that a plurality of roads intersect complicatedly within a region of about 100 m in radius. In this case, since there are a plurality of intersections and they are intersecting diagonally and so on, even if the color of the guide route and the color of the ordinary roads are made different and an intersection at which route guidance to be provided is displayed enlarged, when the vehicle actually travels through the vicinity of that intersection it is difficult for the driver to make out the exact road to be travelled. That is, there is a possibility that the driver can not distinguish which of the roads intersecting complicatedly within the above-mentioned region of about 100 m in radius the guide route is indicating.